Switching power supply

ABSTRACT

A switching power supply apparatus ( 200 ) for controlling the operation of a switching FET ( 225 ) adapted for switching a rectified smoothed output of a primary side rectifying smoothing circuit ( 215 ) by a switching controlling circuit ( 230 ) having a hysteresis low-voltage mistaken operation prohibiting circuit. An output of a ternary winding ( 220 C) of a converter transformer is rectified and smoothed by a rectifying smoothing circuit ( 238 ) to drive the switching controlling circuit ( 230 ). An output voltage of the ternary winding ( 220 C), varied in dependence upon the load state on the secondary side of a converter transformer ( 220 ) is set so as to be lower than a low voltage protective voltage in case the voltage is lower than a design load and so as to be higher than the low voltage protective voltage in case the voltage is higher than the design load, in order to carry out an intermittent operation during standby time. Thus, the switching operation during standby time may be carried out intermittently to minimize the power consumption to realize the energy saving during standby time, simply by adjusting the values of respective key devices, without appreciably changing the pre-existing circuitry.

BACKGROUND OF THE INVENTION

This invention relates to a switching power supply apparatus forcontrolling the switching operation of a switching element adapted forswitching a rectified smoothed output of a primary side rectifyingsmoothing circuit by a switching controlling circuit having a hysteresislow-voltage mistaken operation preventative circuit.

BACKGROUND ART

Up to now, a switching power supply apparatus for switching a DCcurrent, obtained on rectifying and smoothing the commercial AC current,at a high frequency on the order of, for example, 100 kHz, and fortransforming the resulting current to a desired voltage by a transformerto a high efficiency, has been in use.

As an output voltage controlling system in the above-described switchingpower supply apparatus, a pule width modulation (PWM) controlling systemfor controlling the duty ratio of the switching pulses responsive tochanges in the output voltage, and a frequency (e.g., resonation)controlling system or a phase controlling system for controlling thefrequency or the phase of switching pulses, have so far been used.

FIG. 1 shows an illustrative circuit structure of a conventionalswitching power supply apparatus employing a PWM controlling system.

This switching power supply apparatus 100 includes a primary rectifyingsmoothing circuit 115 for rectifying and smoothing the AC input suppliedfrom a commercial power supply AC through an AC filter 110. To thisprimary rectifying smoothing circuit 115, there are connected the drainof a switching FET 125 through a primary winding 120A of a convertertransformer 120 and a power supply terminal 130A of a switchingcontrolling circuit 130 through a startup circuit 140. The switchingcontrolling circuit 130 controls the switching operation of theswitching FET 125 by PWM control. The power supply terminal 130A isgrounded through a capacitor 135.

In order to prevent malfunctions of the switching controlling circuit130 at the time of lowering of the power supply voltage, the switchingcontrolling circuit 130 has enclosed therein a hysteresis low voltagemalfunction prohibiting circuit. When the power supply voltage Vcc,applied to the power supply terminal 130A, is increased from 0V, theprohibiting circuit starts its operation at Vcc=16.5V and, when thepower supply voltage is lowered, the prohibiting circuit interrupts anoutput at Vcc=9.0V.

A secondary rectifying smoothing circuit 150 is connected to a secondarywinding 120B of the converter transformer 120, such that a converteroutput obtained in the secondary winding 120B of the convertertransformer 120 is rectified and smoothed by the secondary rectifyingsmoothing circuit 150 so as to be output via an output filter 155. Anoutput detection circuit 170 is connected to this secondary rectifyingsmoothing circuit 150 through a resistance dividing circuit 160 fordetecting the output voltage and a resistor 165 for detecting the outputcurrent. A detection output by this output detection circuit 170 is fedback via a photocoupler 180 to the switching controlling circuit 130.The output detection circuit 170 and the photocoupler 180 are actuatedby a rectified smoothed output of a rectifying smoothing circuit 190,connected to a secondary winding 120B of the converter transformer 120,as a driving source.

The switching controlling circuit 130 is started by the startup currentsupplied on startup from the primary rectifying smoothing circuit 115,through a startup circuit 140, to commence the supply of switchingpulses to the switching FET 125. After startup, the switchingcontrolling circuit 130 is actuated with the rectified smoothed outputby a rectifying smoothing circuit 138, connected to a ternary winding120C of the converter transformer 120, as a driving power supply. Thatis, the switching controlling circuit PWM-controls the switchingoperation of the switching FET 125 to stabilize the converter output, bythe duty cycle of the switching pulse being changed responsive to thedetection output by the output detection circuit 170 fed back by thephotocoupler 180.

If, in the conventional switching power supply apparatus 100, the powerof the output detection circuit 170 is taken from an output line in theusual constant current taking operation (constant current chargingoperation) for a battery, the range of voltage variations is of anextremely wide width, such that a separate power supply is needed whichis capable of supplying a constant stable voltage in order to assurestabilized control. To this end, the range of voltage variations isdiminished to as small a value as possible by providing a seriesregulator, using a different winding of the same transformer with loosecoupling for use as a power supply relatively insusceptible to loadvariations, or by using a separate rectifying smoothing circuit for thesame winding, in order to provide for stabilized control.

In a power source supply system, in which the power for the outputdetection circuit 170 is supplied by separate rectification from thesame winding of the same transformer, in order to control the output ofthe low power switching power supply performing the intermittentoperation during standby to a constant voltage and a constant current,the power required for control during the switching stop time for theintermittent operation is supplied by the smoothing capacitance of arectifying smoothing circuit 190. This increases the capacity of asmoothing capacitor 191 of the rectifying smoothing circuit 190.Moreover, there is raised a problem of the effect of chronologicalchanges in capacitance because a large capacitance is required and hencean electrolytic capacitor of good volume capacitance ratio is used.

On the other hand, in a conventional standby power saving type switchingpower supply apparatus, an intermittent operation is carried out bydetecting the oncoming no-load or light-load conditions to stop theswitching operation to save the power.

For detecting the load, it is known to insert a resistor in series witha load to detect the voltage drop occurring across both ends. If theminute current for the light-load state (of the order of 10 mA) is to bedetected accurately by this method, the detection resistance must be setto tens to hundreds of ohms. In the case of a heavy load, the voltagedrop or heat evolution at the detection resistor poses a problem.Heretofore, these problems are tackled by a method of shorting thedetection resistor with a semiconductor device. However, the circuitrybecomes complicated to raise the cost.

If the state of the load is detected and found to be a normal load, theLED of the photocoupler for verifying the load state is turned on andthe resulting signal is transmitted to a primary side control circuit.If the state of the load is found to be no load or light load state, theLED of the photocoupler is turned off to stop the switching. In order toperform this control, it is necessary to effect the transmission using aphotocoupler for verifying the load state distinct from the feedbackphotocoupler for controlling the constant voltage, thus requiring aredundant circuit.

In startup, the photocoupler for verifying the load state experiences anoutput devoid state and hence the driving voltage is in shortage becausecertain time is needed until the secondary side output voltage isincreased to a setting value. As this state tends to be judged to be theno-load or light-load state, the circuitry for avoiding the mistakenjudgment must needs be added.

Moreover, the photocoupler is on at all times during the normaloperation, thus consuming redundant power, with the result that powersaving during the operation is not achieved.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide aswitching power supply apparatus in which the standby time switchingoperation is performed intermittently, by simply adjusting the values ofrespective key devices, without appreciably changing the pre-existingcircuit, to minimize the power consumption to achieve energy savingduring the standby time, as well as to enable ordinary operations, suchas constant voltage constant current operations or various protectivefunctional operations, without being affected by the circuitry designedto perform the intermittent operations.

The present invention provides a switching power supply apparatuswherein a startup current from a primary side startup circuit issupplied to a switching controlling circuit having a hysteresislow-voltage mistaken operation prohibiting circuit to start up theswitching controlling circuit by the energy accumulated in a capacitorfor a voltage range from a low voltage protective voltage of thelow-voltage mistaken operation prohibiting circuit to a release voltage,a switching operation of a switching element switching a rectifiedsmoothed output of a primary side rectifying smoothing circuit, suppliedto a primary side of a converter transformer, is controlled by theswitching controlling circuit, an output of a ternary winding of theconverter transformer is rectified and smoothed after startup, by arectifying smoothing circuit to produce a rectified smoothed outputwhich drives the switching controlling circuit, a converter output,obtained in a secondary winding of the converter transformer, isrectified and smoothed by a secondary side rectifying smoothing circuit,so as to be output, an error signal is fed back from a secondary sideoutput detection circuit through a photocoupler to the switchingcontrolling circuit to control the switching operation of the switchingelement by the switching controlling circuit, and wherein an outputvoltage in the ternary winding, changing depending on the load state ina secondary side of the converter transformer, is set so as to be lowerthan the low voltage protective voltage for the current less than asetting load current and so as to be higher than the low voltageprotective voltage for the current not less than the setting loadcurrent, whereby an intermittent operation is performed during standbytime.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of a conventionalswitching power supply apparatus.

FIG. 2 is a block diagram showing the structure of a switching powersupply apparatus according to the present invention.

FIG. 3 is a waveform diagram showing the waveform of the intermittentoperation of the switching power supply apparatus according to thepresent invention.

FIG. 4 is a waveform diagram showing the secondary side output waveformduring the intermittent operation of the switching power supplyapparatus according to the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Referring to the drawings, a present embodiment of the present inventionis explained in detail.

The present invention is applied to a switching power supply apparatus200 having a structure shown for example in FIG. 2.

This switching power supply apparatus 200 includes a primary siderectifying smoothing circuit 215, for rectifying and smoothing the ACinput supplied from the commercial power supply AC through an AC filter210. To this primary side rectifying smoothing circuit 215 is connectedthe drain of a switching FET 225 through a primary winding 220A of aconverter transformer 220.

There is also connected a switching controlling circuit 230 for PWMcontrolling the switching operation of the switching FET 225. Thejunction point of the AC filter 210 and the primary side rectifyingsmoothing circuit 215 is connected through a startup circuit 240 to apower supply terminal 230A of the switching controlling circuit 230.

The power supply terminal 230A of the switching controlling circuit 230is supplied with a rectified smoothed output by a rectifying smoothingoutput 238, connected to a ternary winding 220C of the convertertransformer 220, as a driving power. The power supply terminal 230A isgrounded via a capacitor 235.

In order to prevent mistaken operations in case the power supply voltageis lowered, the switching controlling circuit 230 has enclosed therein ahysteresis low voltage mistaken operation prohibiting circuit, suchthat, when the power supply voltage Vcc, applied to the power sourceterminal 230A, is increased from 0V, the operation is initiated atVcc=16.5V, with the control output being interrupted at Vcc=9.0V whenthe power supply voltage is lowered.

On the other hand, the switching controlling circuit 230 has a softstart function, specifically, a CS terminal 230B for soft start controlis grounded via a capacitor 231 affording a time constant for softstart, while being connected to the power supply terminal 230A through aZener diode 232 adapted for detecting the power supply terminal 230A.

The switching controlling circuit 230 has an over-current limitingfunction, and includes an IS terminal 230C for over-current detection,which is connected through a resistor for correcting the input voltageto a junction between the primary side rectifying smoothing circuit 215and the primary winding 220A of the converter transformer 220 and whichis also connected to a constant power protection circuit 234, made up bythree resistors 234A, 234B and 234C, connected to the source of theswitching FET 225.

The startup circuit 240 includes a constant current circuit 241,connected to a junction between the AC filter 210 and the primary siderectifying smoothing circuit 215, and which is connected through areverse current inhibiting diode 248 to a power supply terminal 230A ofthe switching controlling circuit 230.

The constant current circuit 241 includes first and second NPNtransistors 244, 245, having collectors connected via resistors 242, 243to a junction between the AC filter 210 and the primary side rectifyingsmoothing circuit 215. The base of the first NPN transistor 244 isconnected to the collector of the second NPN transistor 245. Thejunction between the emitter of the first NPN transistor 244 and thebase of the second NPN transistor 245 is connected via a currentdetection resistor 246 to the emitter of the second NPN transistor 245,while being connected to the cathode of the reverse current inhibitingdiode 248.

In the constant current circuit 241, the voltage across both ends of thecurrent detection resistor 246 is detected by the second NPN transistor245 to control the current flowing from the resistor 243 to the base ofthe first NPN transistor 244 to cause the constant current Ic to flowthrough the current detection resistor 246.

To a secondary winding 220B of the converter transformer 220, there areconnected a rectifying smoothing circuit 252 for supplying the drivingpower to a secondary rectifying smoothing circuit 250 and an outputdetection circuit 270 and a rectifying smoothing circuit 254 forsupplying the driving power to the photocoupler 280. An output end ofthe secondary rectifying smoothing circuit 250 is connected to an outputend of the rectifying smoothing circuit 252 through a diode 253.

A converter output, obtained in a secondary winding 220B of theconverter transformer 220, is rectified and smoothed by the secondaryside rectifying smoothing circuit 250 and output via output filter 255.To the secondary rectifying smoothing circuit 250 is connected theoutput detection circuit 270 via a resistance dividing circuit 260 fordetecting the output voltage and a resistor 265 for detecting the outputcurrent, with the detected output by this output detection circuit 270being fed back to the switching controlling circuit 230 via photocoupler280.

In the above-described switching power supply apparatus 200, theswitching controlling circuit 230 is started by being fed in startup viastartup circuit 240 to begin to supply the switching pulses to theswitching FET 225. After startup, the switching controlling circuit 230is driven with the rectified smoothed output by the rectifying smoothingcircuit 238 connected to the ternary winding 220C of the convertertransformer 220 as the driving power. The detection output by the outputdetection circuit 270 is fed back via photocoupler 280 to PWM-controlthe switching operation of the switching FET 225 to provide for astabilized converter output.

In startup, the switching power supply apparatus 200 operates asfollows:

When the AC input is supplied from the commercial power supply, theconstant current (Ic=0.1 mA) is caused to flow through the resistor 242of the startup circuit 240, first NPN transistor 244, current detectionresistor 246 and the reverse current inhibiting diode 248 to thecapacitor 235 to start the charging.

The voltage Vcc, applied to the power supply terminal 230A of theswitching controlling circuit 230, is increased gradually as thecharging of the capacitor 235 proceeds. When the minimum startup voltageof the low voltage mistaken operation prohibiting circuit (16.5V) isexceeded, the switching controlling circuit 230 commences its operationto output the switching pulses to the switching FET 225. The currentconsumption of the switching controlling circuit 230 is increased atthis time, with the voltage across the terminals of the capacitor 235being lowered. By the operation of the low voltage mistaken operationprohibiting circuit with hysteresis, the switching operation iscontinued by the energy stored in the capacitor 235 up to the minimumoperating voltage (V1=9V).

By the current switched in the interim, the high frequency current iscaused to flow via converter transformer 220 through the secondary andternary windings 220B, 220C. This high frequency current is rectified bythe secondary rectifying smoothing circuit 250 and output via outputfilter 255 from the output terminal as a converter output.

This voltage is also compared to a reference voltage via resistancedividing circuit 260 by the output detection circuit 270. If the outputvoltage is high or low, a light-emitting diode 280A of the photocoupler280 is turned on or off, respectively, to transmit the signal to theprimary side switching controlling circuit 230 to vary the duty of theswitching pulse supplied to the gate of the switching FET 225 to controlthe output voltage Vout to a preset voltage.

On the other hand, the output of the ternary winding 220C is rectifiedand smoothed on the primary side by the rectifying smoothing circuit 238and charged to the capacitor 235 so as to be supplied as the drivingpower supply for the switching controlling circuit 230. Since thevoltage Vcc (12V under the normal operating state) is higher than thevoltage from the startup circuit 240 (11V on stabilized startup), thepower supply from the startup circuit 240, connected via the reversecurrent inhibiting diode 248, is halted.

With the secondary winding 220B and the ternary winding 220C of theconverter transformer 220 wound in the opposite direction to that of theprimary winding 220A, the switching power supply apparatus 200 is aswitching power supply of the on/off (flyback) switching system. If theload of the controlled secondary winding 220B is heavy, the outputcharacteristics of the non-controlled ternary winding 220C becomecorrespondingly higher, termed below the characteristics ofcross-regulation, while being independent from the input voltage appliedto the primary winding 220A and being constant for the constant load ofthe secondary winding 220B.

During the normal operation, the switching power supply apparatus 200operates as follows:

In the present switching power supply apparatus 200, an error signal,obtained on comparing the output voltage to the reference voltage by thesecondary side output detection circuit 270 after startup, is fed backthrough the photocoupler 280 to an FB terminal 230D for feedback inputof the primary side switching controlling circuit 230 to commence theswitching control of the switching FET 225 by the switching controllingcircuit 230. The power supply terminal 230A of the switching controllingcircuit 230 is supplied with a driving power from the rectifyingsmoothing circuit 238 connected to the ternary winding 220C of theconverter transformer 220. The switching controlling circuit 230performs PWM control of the switching operation of the switching FET 225so that the output voltage Vout will be constant against non-loadoperations or changes in the input voltage. When more than a designquantity of the load current is taken from the output, the voltageacross both ends of the current detection resistor 265 is higher thanthe design reference value and is detected by the output detectioncircuit 270 adapted for comparing the voltage across both ends of thecurrent detection resistor 265 to the reference voltage. The switchingcontrolling circuit 230 is responsive to a detection output by thesecondary side output detection circuit 270 to lower the output voltageVout to execute PWM control of the switching operation of the switchingFET 225.

Although the output voltage Vout is lowered at this time, the powersupply voltage of the output detection circuit 270, supplied from therectifying smoothing circuit 252 distinct from the secondary rectifyingsmoothing circuit 250, is not lowered as compared to the output voltageVout, thus enabling stabilized control. On the other hand, the outputvoltage of the rectifying smoothing circuit 238, connected to theternary winding 220C of the converter transformer 220, is higher thanthe minimum operating voltage of the low-voltage mistaken operationprohibiting circuit (V1=9V), such that the switching controlling circuit230 is able to continue the stabilized operation with the output voltageof the rectifying smoothing circuit 238 as the driving power supply.

In a no-load condition, the switching power supply apparatus 200 alsooperates as follows:

In the present switching power supply apparatus 200, an error signal,obtained on comparing the output voltage to the reference voltage by thesecondary side output detection circuit 270 after startup is fed backthrough the photocoupler 280 to an FB terminal 230D for feedback inputof the primary side switching controlling circuit 230 to commence theswitching control of the switching FET 225 by the switching controllingcircuit 230. Due to lag in the transient response or to there being noload, the output voltage Vout, generated in the secondary side, ishigher than the reference voltage used for comparison in the outputdetection circuit 270. The result is that the output to the lightemitting diode 280A of the photocoupler 280 is turned on to actuate theswitching controlling circuit 230 to halt the switching operation of theswitching FET 225. Although an output voltage is generated in theinterim across the ternary winding 220C, such voltage is lower than thelow voltage protective voltage, because of the light output load, thisoutput voltage being insufficient to raise the power supply voltage Vccof the switching controlling circuit 230. The power supply voltage Vccof the switching controlling circuit 230 is lowered to a level of theminimum operating voltage of the low-voltage mistaken operationprohibiting circuit (V1=9V). When the power supply voltage Vcc islowered to 9V, the switching controlling circuit 230 halts its operationto enter into a standby state. In the standby state, the currentconsumption of the switching controlling circuit 230 is decreased (6 μA)to increase the power supply voltage Vcc of the switching controllingcircuit 230 through the startup circuit 240. When the power supplyvoltage Vcc exceeds the minimum startup voltage (16.5 V) of thelow-voltage mistaken operation prohibiting circuit, the primary sideswitching controlling circuit 230 wakes up immediately to cause a PWMswitching operation of the switching FET 225. This switching powersupply apparatus 200 reiterates the aforementioned intermittentoperating states, as shown in FIG. 3, to suppress the power consumptionunder a no-load condition.

It is noted that the low-voltage mistaken operation prohibiting circuit,with the operating voltage of 16.5V, enclosed within the switchingcontrolling circuit 230, has hysteresis characteristics, such that ittakes some time until the operation commencing voltage is reached. Thesecondary side output detection circuit 270 is continuing its operationin the interim by the energy stored in the capacitor 252A of therectifying smoothing circuit 252. When the voltage is gradually lowereduntil the potential difference between it and the output exceeds theforward voltage Vf of the diode 253 exceeds the forward voltage Vf ofthe diode 253, the diode 253 is turned on so that the output detectioncircuit 270 continues to be supplied with the energy stored in thecapacitors 250A and 250B of the secondary rectifying smoothing circuit250. The rectified smoothed output by the secondary rectifying smoothingcircuit 250, that is the secondary output voltage Vout, is also loweredin the interim, such that the voltage supplied from the rectifyingsmoothing circuit 254 to the light emitting diode 280A of thephotocoupler 280 becomes equal to or lower than a limit value (5 V), asshown in FIG. 4. This decreases the current flowing through the lightemitting diode 280A so that a phototransistor 280B of the photocoupler280 is in the high impedance state. The period of the intermittentoperation can be controlled from the secondary side by suitablyselecting the capacitances of the capacitors 250A, 250B, 252A and 254Aor by interconnecting plural diodes 254B of the rectifying smoothingcircuit 254 in series, for thereby adjusting the forward voltage value.

It should be noted that transistor switches or semiconductor switchesmay be used in place of the diode 253 supplying the power from thesecondary rectifying smoothing circuit 250 to the output detectioncircuit 270 in the course of the standby intermittent operations.

Meanwhile, under the light load condition, this switching power supplyapparatus 200 executes the following intermittent operations:

That is, in this switching power supply apparatus 200, an error signalobtained on comparing the output voltage to the reference voltage by thesecondary side output detection circuit 270 after startup is fed back toan FB terminal 230D for feedback input of the primary side switchingcontrolling circuit 230 through the photocoupler 280 to commence the PWMcontrol of the switching operations of the switching FET 225 by theswitching controlling circuit 230 to stabilize the output voltage Voutgenerated in the secondary side. If the load is light, the voltage ofthe driving supply power supplied to the primary side switchingcontrolling circuit 230 from the ternary winding 220C of the convertertransformer 220 through the rectifying smoothing circuit 238, isdecreased under the effect of the cross-regulation.

Thus, in the present switching power supply apparatus 200, the outputvoltage of the ternary winding 220C of the converter transformer 220 isadjusted as follows:

The output voltage of the ternary winding 220C of the convertertransformer 220 is set, depending on the number of windings and thedegree of linkage thereof and on the resistance value of the resistor236, to a voltage not larger than the minimum operating voltage (V1=9V)of the low-voltage mistaken operation prohibiting circuit of theswitching controlling circuit 230. This voltage not larger than theminimum operating voltage is 10.1V taking into account the forwardvoltage Vf of the diodes 235A and 235B of the rectifying smoothingcircuit 238. In the case of the load current not less than the normalload current, such as approximately 10 mA, the output voltage of theternary winding 220C is set to not less than a voltage for which thelow-voltage mistaken operation prohibiting circuit is not in operation(10.2 V).

By setting the output voltage of the ternary winding 220C of theconverter transformer 220, as described above, the supply of the currentconsumed in the switching controlling circuit 230 falls into shortage,with the power supply voltage Vcc of the switching controlling circuit230 being progressively lowered to the minimum operating voltage forwhich the low-voltage mistaken operation prohibiting circuit (V1=9V) isin operation. When the power supply voltage Vcc is lowered to 9V, theswitching controlling circuit 230 halts its operation to enter into astandby state. In the standby state, the current consumption of theswitching controlling circuit 230 is decreased (6 μA) to increase thepower supply voltage Vcc of the switching controlling circuit 230through the startup circuit 240. When the power supply voltage Vccexceeds the minimum startup voltage (16.5 V) of the low-voltage mistakenoperation prohibiting circuit, the primary side switching controllingcircuit 230 wakes up immediately to cause a PWM switching operation ofthe switching FET 225. This switching controlling circuit 230 reiteratesthe aforementioned intermittent operating states to suppress the powerconsumption under a light load condition.

If once the primary side switching FET 225 commences the switchingoperation, the primary side switching FET 225 continues its switchingoperation until the secondary side output detection circuit 270 detectsthat the secondary side output voltage Vout is increased to a value notless than a prescribed value and the resulting detection output is fedback as a switching halting signal via photocoupler 280 to the FBterminal 230D for feedback input of the primary side switchingcontrolling circuit 230 through the photocoupler 280.

As the aforementioned switching halting signal is fed back to the FBterminal 230D for feedback input of the primary side switchingcontrolling circuit 230, the switching operation of the switching FET225 is halted, such that the voltage V1 supplied from the capacitor 252Aof the secondary side rectifying smoothing circuit 252 to the outputdetection circuit 270 or the voltage V2 supplied from the capacitor 254Aof the rectifying smoothing circuit 254 to the light emitting diode 280Aof the photocoupler 280 is decreased progressively. When the voltage V2supplied to the light emitting diode 280A falls to an operating limitvalue (5V) or lower, the current flowing through the light emittingdiode 280A is decreased, with the phototransistor 280B of thephotocoupler 280 being in a high-impedance state, thus in a state readyfor a switching operation of the primary intermittent period. Thisestablishes the uncontrolled state similar to that at the startup timepoint, such that, during the switching operation of the next primaryintermittent period, the switching controlling circuit 230 commences theswitching operation of the switching FET 225 by the PWM signal of themaximum width as determined by the upper limit of the soft start. Whenthe switching FET 225 commences the switching operation, the chargingcurrent flows through diode 254B to a capacitor 254A of the secondaryside rectifying smoothing circuit 254. The voltage V2 supplied from therectifying smoothing circuit 254 to the light emitting diode 280A of thephotocoupler 280 is raised to a value not lower than the operating limitvalue (5V) of the light emitting diode 280A to establish the operatingstate of the photocoupler 280. Thus, an error signal obtained oncomparing the output voltage to the reference voltage by the secondaryside output detection circuit 270 is fed back to an FB terminal 230D forfeedback input of the primary side switching controlling circuit 230through the photocoupler 280 to commence the PWM control of theswitching operations of the switching FET 225 by the switchingcontrolling circuit 230.

It is noted that the capacitance of the capacitor 254A of the rectifyingsmoothing circuit 254 is set so that the time until the voltage V2supplied from the secondary side rectifying smoothing circuit 254 to thelight emitting diode 280A of the photocoupler 280 is lowered to a valuenot larger than the operating limit value (5V) of the light emittingdiode 280A will be shorter than the primary intermittent period.

In general, the power consumption of the light-emitting diode is largerthan that of the output detection circuit 270, in particular the ICprepared by CMOS, such that, by separating the rectifying smoothingcircuit 254 for supplying the driving power to the light emitting diode280A of the photocoupler 280 from the rectifying smoothing circuit 252for supplying the driving power to the output detection circuit 270 andby decreasing the capacitance of the capacitor 254A of the rectifyingsmoothing circuit 254, it is similarly possible to decrease the suppliedvoltage more promptly as compared to the controlling system before thesecondary output voltage Vout is excessively lowered, and to shorten theintermittent period to reduce the output ripple by using a highimpedance photocoupler 280.

Moreover, this switching power supply apparatus 200 transfers from theintermittent operation in the non-load or light-load state to the normaloperation.

That is, in transferring from the intermittent operation under thenon-load or light-load state to the normal operation, the load currenttaken out from the secondary winding 220B of the converter transformer220 is increased when the intermittent switch is on. The on-time of thelight emitting diode 280A of the photocoupler 280 by the increasingoutput voltage becomes shorter, with the light emitting diode 280A beingoff in short time. The output voltage of the ternary winding 220C of theconverter transformer 220 is also increased with the increasing loadcurrent, with the power supply voltage Vcc of the primary side switchingcontrolling circuit 230 not being lowered to the minimum operatingvoltage (V1=9V) of the low-voltage mistaken operation prohibitingcircuit, whereby the switching controlling circuit 230 outputs switchingpulses to perform PWM control of the switching operation of theswitching FET 225 to provide for the normal continuous operation of theconstant voltage output.

The switching power supply apparatus 200 transfers from the normaloperation to the intermittent operation as follows:

That is, in transferring from the normal operation to the intermittentoperation under non-load or light-load, the output voltage of theternary winding 220C of the converter transformer 220 is decreased withthe decreasing load. When the power supply voltage Vcc of the primaryside switching controlling circuit 230 is lowered to the minimumoperating voltage of the low-voltage mistaken operation prohibitingcircuit (V1=9V), the switching controlling circuit 230 halts itsoperation to enter into a standby state. In the standby state, thecurrent consumption of the switching controlling circuit 230 isdecreased (6 μA), with the constant current being caused to flow fromthe startup circuit 240 through the reverse current prohibiting diode248 to the capacitor 235, by way of charging, to increase the powersupply voltage Vcc of the switching controlling circuit 230. When thepower supply voltage Vcc exceeds the minimum startup voltage (16.5 V) ofthe low-voltage mistaken operation prohibiting circuit, the primary sideswitching controlling circuit 230 wakes up immediately to cause a PWMswitching operation of the switching FET 225. This switching controllingcircuit 230 reiterates the aforementioned intermittent operating statesto suppress the power consumption under a light load condition.

Although the present invention is applied to a switching power supplyapparatus, employing the PWM controlling system, the present inventionmay be applied to a switching power supply apparatus employing afrequency controlling system.

Thus, according to the present invention, the switching operation duringstandby may be carried out intermittently, by simply adjusting the valueof the respective key devices, without appreciably changing the circuitnow in use, such as to achieve energy saving during standby, as well asto perform usual operations, such as constant voltage, constant currentand various protective functional operations, without affecting thecircuitry designed to execute the intermittent operations.

Moreover, the driving power during the normal operations of the primaryside switching controlling circuit is supplied from the ternary winding,with the voltage being proportionate to the secondary side outputcurrent, by the cross-regulation effect, so that, by setting the sleepstate or the wake-up state by exploiting the low-voltage mistakenoperation prohibiting circuit of the primary side switching controllingcircuit, the intermittent operating state can readily be achieved underthe cross-regulation effect.

The intermittent period can be controlled by setting the capacitance,bresistance and the number of turns of the windings, thus providing asimplified circuit.

The intermittent period can be adjusted readily in stability.

Moreover, the output ripple voltage adjustment by the intermittentoperation can be adjusted extremely readily.

1. A switching power supply apparatus comprising: a switching control circuit having an input voltage and a hysteresis low-voltage mistaken operation circuit; a rectifying smoothing circuit for producing a driving output for driving said switching control circuit; a capacitor for accumulating energy corresponding to a range of voltages of said hysteresis low-voltage mistaken operation circuit from a protective voltage to a release voltage; a primary side startup circuit for supplying a startup current to said switching control circuit, wherein said hysteresis low-voltage mistaken operation circuit prohibits said primary side startup circuit from starting up said switching control circuit based on energy accumulated in said capacitor; a primary side rectifying circuit having a primary rectified output; a secondary side rectifying circuit having a secondary rectified output; a converter transformer having a primary winding which receives said primary rectified output, a tertiary winding which is wound in a direction opposite to that of the primary winding and has a tertiary output, and a secondary winding wound in a direction opposite to that of the primary winding; a switching element for switching said primary rectified output; and an output detection circuit comprising an error feed back signal to said switching control circuit via a photocoupler to control the switching element by said switching control circuit; wherein after said primary side startup circuit supplies said startup current to said switching control circuit, a tertiary output of said tertiary winding is rectified and smoothed by said rectifying smoothing circuit to produce said driving output; wherein said tertiary output is lower than said protective voltage when a current supplied to said secondary winding is less than a predetermined setting load current, and said tertiary output is higher than said protective voltage when a current into said secondary winding is greater than said predetermined setting load current; and wherein the switching control circuit enters into a standby state when said input voltage is less than or equal to said release voltage.
 2. The switching power supply apparatus according to claim 1 wherein said output detection circuit is driven by said secondary rectified output, and wherein a supplementary circuit is provided for supplying power to said output detection circuit from said secondary side rectifying circuit during said standby state.
 3. The switching power supply apparatus according to claim 2 wherein said photocoupler is driven by said secondary rectified output. 